Metal gate stack with etch stop layer having implanted metal species

ABSTRACT

A metal gate structure and method of forming the same introduces metal impurities into a first metal layer, made of TiN, for example. The impurities create a surface region of greater etch selectivity that prevents overetching of the TiN during the etching of an overlying tungsten gate during the formation of the metal gate structure. The prevention of the overetching of the TiN protects the gate oxide from undesirable degradation. The provision of aluminum or tantalum as the metal impurities provides adequate etch stopping capability and does not undesirably affect the work function of the TiN.

RELATED APPLICATIONS

[0001] The present invention contains subject matter similar to thatdisclosed in U.S. application Ser. No. ______, filed on ______ (AttorneyDocket No. 50432-112).

FIELD OF THE INVENTION

[0002] The present invention relates to the field of semiconductorprocessing, and more particularly to the formation of metal gateelectrodes.

BACKGROUND OF THE INVENTION

[0003] In the integrated circuit (IC) industry, metal-oxidesemiconductor (MOS) transistors have typically been formed utilizingpolysilicon gate electrodes. Polysilicon material has been preferred foruse as an MOS gate electrode due to its thermal resistive properties(i.e., polysilicon can better withstand subsequent high-temperatureprocessing). Polysilicon's robustness during high-temperature processingallows polysilicon to be annealed at high temperatures along with sourceand drain regions. Furthermore, polysilicon's ability to block the ionimplantation of doped atoms into a channel region is advantageous. Dueto the ion implantation blocking potential of polysilicon, polysiliconallows for the easy formation of self-aligned source and drainstructures after gate patterning is completed.

[0004] However, polysilicon gate electrodes have certain disadvantages.For example, polysilicon gate electrodes are formed from semiconductormaterials that suffer from higher resistivities than most metalmaterials. Therefore, polysilicon gate electrodes may operate at muchslower speeds than gates made of metallic materials. To partiallycompensate for the high resistance, polysilicon materials often requireextensive and expensive silicide processing in order to increase theirspeed of operation to acceptable levels.

[0005] Metal gates are therefore being investigated as replacements forpolysilicon gates. Metal gates are fabricated in a manner that issimilar to the fabrication processes for polysilicon gates. An exemplarylayer structure is depicted in FIG. 1A of a metal gate structure. Gateoxide layer 12 is first deposited on a substrate 10. A barrier layer 14,made of titanium nitride (TiN), for example, is formed on the gate oxidelayer 12. The layer 14 is primarily chosen for appropriate workfunctionproperties which determine the threshold voltage of the transistorstructure. The barrier layer also aids in the adhesion of thesubsequently formed metal gate. The TiN can be deposited by conventionalmethodologies, such as physical vapor deposition (PVD). Alternatematerials such as TaN, TaS_(x)N_(y), WN etc. may be used for thispurpose

[0006] A metal gate layer 16 is then formed on the barrier layer 14. Anexemplary material for the metal gate layer 16 is tungsten, althoughother materials may be used. The tungsten is deposited by conventionalmethodologies, such as physical vapor deposition.

[0007] A SiRN anti-reflective coating (ARC) 18 is formed on the metalgate layer 16. This is followed by formation of a cap layer 20 over theARC layer 18. The cap layer 20 may comprise silicon nitride (SiN), forexample. The anti-reflective coating 18 and the cap layer 20 aid in thepatterning of the gate prior to the reactive ion etch process used toform the gate. Anti-reflective coatings 18, 20 increase the resolutionduring the lithography process.

[0008] After the deposition of the layers 12-20 over the substrate 10,the metal gate is now etched. This is accomplished by conventionalpatterning and etching techniques. The tungsten layer is typicallyetched with a fluorine containing chemistry, such as SF₆/N₂ orSF₆/Cl₂/N₂, with WF₆ being the primary product species. The latterchemistry has yielded good profiles. In the latter case, an appropriateSF₆/Cl₂ ratio may be chosen to provide the best profiles. The recipe mayeven be richer in Cl₂ than in SF₆ as required. It is desirable for theetchant to have good selectivity to the TiN of the barrier layer 14 sothat the tungsten can be cleared across the entire wafer withoutattacking the gate oxide. Hence, the TiN ideally serves as an etch stoplayer during the etching of the tungsten. An ideal etching process isdepicted in FIG. 1B, which shows the patterning of the metal gateelectrode by an anisotropic reactive ion etch process, stopping on theTiN at the barrier layer 14. However, this depiction is only an idealdepiction, as the TiN has proven in practice to be an inadequate etchstop layer. As depicted in FIG. 1C, when the tungsten is being clearedfrom the rest of the wafer, the TiN is completely etched on some partsof the wafer (indicated by reference numeral 22 in FIG. 1C) allowing theetchant to attack the gate oxide 12. This occurs because TiN readilyetches in the Cl₂ containing W etch chemistry. This results in the gateoxide being exposed either to the F from the W chemistry or beingsubject to the Cl-based TiN chemistry for the course of the TiN etch,both of which result in damage to the gate oxide. The use of endpointmonitors such as optical emission from W species to stop the W etch fromproceeding once the W film clears also does not reliably solve thisproblem, since the thin TiN film continues to etch quickly while theendpoint is being detected. Thus, even though a TiN etch selective togate oxide may be employed when W endpoint is detected, the attack ofTiN during the W etch process itself makes this approach unreliable inpractice. Simply increasing the TiN thickness itself is not practicalowing to increases in stress leading to possible delamination and/or anincrease in sheet resistance. The complete etching away of the TiNduring the tungsten etch leads to degraded gate oxide and decreasedyield.

[0009] Replacing the TiN with different etch stop material maydetrimentally affect the work function of the TiN, and such etch stopmaterial may not exhibit the adhesion properties that are desirable inthe TiN. However, there is a need for improved structure that allows theetching of tungsten with a Cl₂/SF₆/N₂ process that properly stops on theetch stop layer and protects the gate oxide across the wafer, withoutdetrimentally affecting the work function of the metal gate.

SUMMARY OF THE INVENTION

[0010] This and other needs are met by embodiments of the presentinvention which provide a method of forming a metal gate on a wafer,comprising the steps of forming a gate oxide on a substrate and forminga first metal layer on the gate oxide. The etch selectivity in at leasta surface region of the first metal layer is increased. A second metallayer is formed on the first metal layer. The second metal layer is thenetched to form a metal gate, with the etching stopping on the surfaceregion of the first metal layer.

[0011] In certain embodiments of the invention, the first metal layercomprises TiN and the etch selectivity is increased by implanting ametallic species into the TiN. In certain embodiments of the invention,the metallic species comprises either aluminum or tantalum, depending onthe nature of the W etch chemistry. If the W etch chemistry is F-rich,aluminum may be used as the etch stop layer owing to the low vaporpressure of AlF₃. On the other hand, if the W etch chemistry is Cl-rich,the much lower vapor pressure of TaCl₅ as opposed to TaF₅, WF₆ and TiCl₄will also result in a significant slowdown of the etch rate, allowingthe etch to be terminated when clearing of W is detected. The surfaceregion of the TiN, containing the implanted metallic species, has betteretch selectivity than the region of TiN that does not contain themetallic species. The etch selectivity is thereby improved withoutadditional layers, and without significantly affecting the work functionof the TiN. Hence, the etching of the tungsten may proceed and stop onthe TiN layer, thereby assuredly protecting the gate oxide underlyingthe etch stop layer and the TiN of the first metal layer. A similarapproach may also be used when TaN, TaSiN or WN for example are used asthe underlying metal gates, since the F-component of the W etch willreadily attack these materials as well as gate oxide. The use of analuminum implanted layer will provide a chance to switch to Cl₂ basedetches with suitable additives such as HBr, O₂ or N₂, where the etchrate of these materials is much lower and also results in increasedselectivity to gate oxide materials.

[0012] The earlier stated needs are also met by another embodiment ofthe present invention, which provides a metal gate structure comprisinga gate oxide and a first metal layer on the gate oxide. The first metallayer has a surface region with greater etch selectivity than aremaining region of the first metal layer. A second metal layer is onthe first metal layer. In certain embodiments of the invention, thefirst metal layer comprises TiN, and the second metal layer comprisestungsten. In certain embodiments, the TiN has implanted metal in thesurface region, this metal comprising tantalum in some embodiments andaluminum in other embodiments.

[0013] The earlier stated needs are also met by further embodiments ofthe present invention, which provide a metal gate structure comprising aTiN layer having a lower region and a surface region with metalimpunties. The surface region has greater etch selectivity than thelower region. A tungsten gate is provided on the TiN layer.

[0014] The foregoing and other features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1A depicts a metal gate stack prior to the patterning of themetal gate, in accordance with the prior art.

[0016]FIG. 1B depicts a metal gate after an ideal etching process inaccordance with the prior art.

[0017]FIG. 1C depicts the metal gate after an actual etching process,exhibiting areas of degraded gate oxide, in accordance with methods ofthe prior art.

[0018]FIG. 2 is a depiction of a cross-section of a portion of a metalgate structure during formation of the metal gate, in accordance withembodiments of the present invention.

[0019]FIG. 3 depicts the structure of FIG. 2 after implantation of ionsinto a surface region of a first metal layer of the metal gatestructure, in accordance with embodiments of the present invention.

[0020]FIG. 4 depicts the structure of FIG. 3 following the deposition ofthe metal gate and anti-reflective coatings, in accordance withembodiments of the present invention.

[0021]FIG. 5 depicts the structure of FIG. 4 after etching has beenperformed to etch the metal gate and first metal layer, in accordancewith embodiments of the present invention.

[0022]FIG. 6 depicts the metal gate structure of FIG. 5 after the firstmetal layer has been removed across the wafer in accordance withembodiments of the present invention.

[0023]FIG. 7 depicts the metal gate structure of FIG. 6 after theanti-reflective coatings have been removed in accordance withembodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention addresses and solves problems related tothe formation of metal gate structures, in particular, to those involvedin the etching of a metal gate causing the possible degradation of gateoxide across a wafer. These and other problems are solved, in part, bythe present invention which increases the etch selectivity of the TiNcurrently used in metal gates. This is achieved, in embodiments of theinvention, by implanting a metallic species, such as aluminum ortantalum, into a surface region of the TiN. In etching the tungsten orother metal of a metal gate, the TiN with a surface region of increasedetch selectivity stops the etching better than the TiN of the prior art.This prevents etching through of the TiN and degrading of the gate oxideacross the wafer. Since implantation is employed to increase the etchselectivity of the TiN, a separate, additional etch stop layer is notrequired, so that the stack height is not appreciably altered. Someincrease of the TiN layer from before may be required to compensate forthe finite thickness of the implanted region, if electrical constraintsdetermine that a certain thickness of unmodified TiN is required todefine the gate structure. The improved TiN layer provides adequate etchstopping capability and the implanted metallic species does notdetrimentally affect the work function of the TiN. The implanted speciesmust itself be etchable in chemistries other than that used for the Wetch. For example, Al can be etched in Cl based chemistries, and Ta canbe cleared either by a short F based breakthrough or a longer Cl basedstep.

[0025]FIG. 2 depicts a portion of a metal gate structure during itsformation in accordance with embodiments of the present invention. Asubstrate 30 is provided with a gate oxide layer 32 by conventionalmethodology. The gate oxide layer 32 has a thickness of between about 15to about 30 Å in embodiments of the present invention. A first metallayer 34 is formed on the gate oxide layer 32. The first metal layer 34may comprise TiN, in certain embodiments of the present invention,although other metals may be used. The TiN is deposited by conventionalmethodologies, such as physical vapor deposition, for example. Thethickness of the first metal layer 34 may be between about 100 to about200 Å in embodiments of the present invention. The first metal layer 34serves as a barrier layer and an adhesion layer, in certain embodiments.In the prior art, the TiN metal layer served the function of an etchstop layer. It has been found, however, that untreated TiN is inadequatein this function as it fails to protect the underlying gate oxide acrossthe wafer during the etching of the overlying metal gate layer.

[0026] In order to improve the etch stopping capability during thetungsten etch and protect the underlying gate oxide across the wafer,the first metal layer 34 is treated to improve the etch selectivity ofat least a surface region of the layer 34. As depicted in FIG. 3, thepresent invention introduces metal impurities 38 into at least a surfaceregion 37 of the first metal layer 34. Depending on the implantenergies, the surface region 37 may extend completely through the firstmetal layer 34. In other embodiments of the invention, such as thatdepicted, the surface region 37 extends only through a portion of thefirst metal layer 34. In such embodiments, a lower region 35 of thefirst metal layer 34 contains little or none of the impurities 38.

[0027] Based upon vapor pressure tables and other considerations, themetallic species 38 introduced into the TiN of the first metal layer 34may be one or more of a number of different materials. Candidates forthe metallic species include aluminum, tantalum, copper, and gold. Ofthese materials, aluminum and tantalum are favored, since copper andgold have deleterious effects on transistors. Aluminum forms a stablenonvolatile fluoride AlF₃ at typical cathode temperatures (50° C.) andcan adequately stop F-containing W etch chemistries. Tantalum chloridehas a much lower vapor pressure than TaF₅ and WF₆ and can appreciablyslow down the etch rate in Cl₂-rich SF₆/Cl₂/N₂ tungsten etchchemistries. Although aluminum and tantalum are described as exemplarymetallic species in the first metal layer 34, other materials orcombinations of materials may be used without departing from the scopeof the present invention.

[0028] In certain embodiments of the present invention, an ionimplantation process is employed to introduce the metallic species 38into the surface region 37 of the first metal layer 34. An exemplary ionimplantation process for aluminum or tantalum metallic species uses abeam current density of between about 20-40 E10 ions/cm². The ions areimplanted, in certain exemplary embodiments, with low power, such asless than 1000 eV. An exemplary power is about 100 eV. These processparameters are exemplary only, and may be changed depending on thematerial of the first metal layer 34, the metallic species 38 to beimplanted, the etch selectivity required, etc., and other parametersknown to those of skill in the art.

[0029] The use of implantation to increase the etch selectivity of asurface region 37 of the first metal layer 34 does not affect the heightof the metal gate stack, increases the etch stopping capability of thefirst metal layer 34, and does not significantly impact the workfunction of the TiN in the first metal layer 34.

[0030] Referring to FIG. 4, the remainder of the metal gate structure isprovided on the first metal layer 34. This includes the metal gate,formed by tungsten deposited by PVD, for example. Tungsten may bedeposited as a second metal layer 38 to a thickness of between about 500to about 1,000 Å. Although tungsten is described as an exemplarymaterial, other metals or metal alloys may be employed in the secondmetal layer 38.

[0031] Anti-reflective coatings, such as an SiRN ARC 40, are provided onthe second metal layer 38. A cap layer 42 is then formed over the ARClayer 40. The ARC layer 40 may be between about 300 to about 1,000 Å.The cap layer 42, which may be silicon nitride (SiN), for example, maybe between about 300 to about 1,000 Å. The anti-reflective coating oflayer 40 and the cap layer 42 aid in the patterning of the metal gatestructure.

[0032] Following the formation and patterning of a resist mask, themetal gate is etched, the results of which are depicted in FIG. 5. Areactive ion etch process, which is an anisotropic etch, is performed.As in conventional methodologies, the tungsten in the second metal layer38 is preferably etched with a Cl₂/SF₆/N₂ process, which currentlyprovides the best tungsten profiles. Such a process, however, hasdifficulties stopping on a conventional TiN layer, such as the firstmetal layer 34 prior to implantation of the metallic species 38.Undesirable complete etching of the TiN on some parts of the wafer wouldlead to degraded gate oxides. The improved etch selectivity of the TiNof the first metal layer 34 in the surface region 37, as provided by thepresent invention, however, prevents this unintended etching through tothe gate oxide layer 32. Hence, the etching process proceeds until thesurface region 37 of the first metal layer 34 is reached. If aluminum isused as the implanted species, the etching effectively stops due toformation of a stable, non-volatile AlF₃-rich layer at typical cathodetemperatures (50° C.). Alternately, if tantalum is used as the implantedspecies while using a Cl₂-rich Cl₂/SF₆/N₂ tungsten etch chemistry, theetching slows down appreciably due to the much lower vapor pressure ofTaCl₅ as compared to TaF₅ or WF₆. This permits termination of the W etchbefore any attack of underlying TiN occurs. The complete etching of theTiN of the first metal layer 34 and degradation of the gate oxide in thegate oxide layer 32 is thereby prevented.

[0033] A different etch chemistry is now employed, as depicted in FIG.6, to remove the first metal layer 34 over the gate oxide 32 in areasnot under the second metal layer 38 of the metal gate. In other words,the first metal layer 34 is etched across the wafer except within themetal gate structure. The etching, however, can be precisely controlledto prevent degradation of the gate oxide. Suitable etchants for etchingthe TiN of the first metal layer 34 are well known to those of ordinaryskill in the art and may be appropriately selected.

[0034]FIG. 7 depicts the metal gate structure of FIG. 6 after the caplayer 42 and the anti-reflective coating 40 have been removed byconventional etching techniques. This leaves a metal gate structure thathas a gate oxide, a PVD layer of TiN with implanted metal impurities,and a metal gate layer. The TiN layer with improved etch selectivityprotects the gate oxide across the wafer during the etching of the metalgate and serves to improve the yield. At the same time, the height ofthe metal gate stack is unchanged and the work function of the TiN isnot deleteriously affected.

[0035] Although the present invention has been described and illustratedin detail, it is to be clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the scope of the present invention being limited only by theterms of the appended claims.

What is claimed is:
 1. A method of forming a metal gate on a wafer,comprising the steps of: forming a gate oxide on a substrate; forming afirst metal layer on the gate oxide; increasing etch selectivity in atleast a surface region of the first metal layer; forming a second metallayer on the first metal layer; and etching the second metal layer toform a metal gate, the etching stopping on the surface region of thefirst metal layer.
 2. The method of claim 1, wherein the first metallayer comprises TiN.
 3. The method of claim 2, wherein the step ofincreasing etch selectivity including implanting a metallic species intothe first metal layer.
 4. The method of claim 3, wherein the metallicspecies implanted into the first metal layer comprises aluminum ortantalum.
 5. The method of claim 4, wherein the aluminum or tantalum isimplanted into the first metal layer with a dose of approximately 20-40E10 ions/cm².
 6. The method of claim 5, wherein the aluminum or tantalumis implanted into the first metal layer with a power less than 1000 eV.7. The method of claim 6, wherein the power is approximately 100 eV. 8.The method of claim 4, wherein the second metal layer comprisestungsten.
 9. The method of claim 8, wherein the etching of the secondmetal layer is by a Cl₂/SF₆/N₂ process.
 10. A metal gate structure,comprising: a gate oxide; a first metal layer on the gate oxide; thefirst metal layer having a surface region with greater etch selectivitythan a remaining region of the first metal layer; and a second metallayer on the first metal layer.
 11. The metal gate structure of claim10, wherein the first metal layer comprises TiN with implanted metal inthe surface region.
 12. The metal gate structure of claim 11, whereinthe metal is aluminum or tantalum.
 13. The metal gate structure of claim12, wherein the second metal layer comprises tungsten.
 14. A metal gatestructure comprising: a TiN layer having a lower region and a surfaceregion with metal impurities, the surface region having greater etchselectivity than the lower region; and a tungsten gate on the TiN layer.15. The metal gate structure of claim 14, wherein the metal impuritiescomprise aluminum.
 16. The metal gate structure of claim 15, wherein themetal impurities comprise tantalum.
 17. The metal gate structure ofclaim 14,wherein the metal impurities are implanted metal ions.
 18. Themetal gate structure of claim 17, wherein the implanted metal ionscomprise aluminum ions or tantalum ions.
 19. The metal gate structure ofclaim 17, wherein the surface region of the TiN has greater etchselectivity to Cl₂/SF₆/N₂ than the surface region.